1. Field of the Invention
The field of the present invention is directed to circularly-polarized antennas and, more particularly, circularly-polarized antennas for a wireless sensor system in an aircraft environment.
2. Description of the Related Art
The aircraft industry has been using copper wires and harnesses to connect sensors and systems since its origination. While there have been many improvements in all aspect of these assemblies, they remain costly, heavy and prone to failures. For instance, the Federal Aviation Administration has recently initiated efforts to standardize and anticipate wire defects to avoid catastrophic failures, reduce wire inspection costs, and improve fleet readiness.
Systems that use substantial sets of wires and harnesses are good candidates to benefit from wireless technology. For instance, some aircraft systems require inputs from a large number of sensors, harnesses, and junction boxes. Some of these sensors are located remotely from the data processors, and throughout the aircraft form tail rotor to cockpit, which require a harness of two dozen or more wires and can weigh twenty or more pounds. It would be beneficial if the aircraft would not have to carry these heavy harnesses. Some rough figures from the aerospace industry estimate a saving of $500 to $1000 can be achieved for each pound reduced off the aircraft weight. The use of wireless sensor technology is one way to reduce the added weight of wires on these aircraft.
Replacing wires with wireless technology is not a simple solution. Fading or the excessive attenuation of a wireless signal through a propagation medium, is a major issue in RF communications, particularly in an aircraft environment where there is often a lot of clutter and obstructing metal structures and equipment that can block or reflect wireless transmission. The presence of deep fading requires excess power margin to be budgeted in the link to mitigate 10-20 dB fading. However, budgeting excess margin increases power consumption, reduces effective range, and may not completely mitigate fading. Although traditional wireless systems employ antenna diversity techniques to mitigate fading, these techniques add cost, size, weight and complexity that is often not practical for wireless sensing systems. As part of improving the RF communications between nodes of the Wireless Sensor (WS) system, antenna polarization is given an important consideration. In particular, the inventors have found that circularly polarized antennas are better than linearly polarized antennas in many circumstances of interest. Depending on the nature of the wireless link, circular polarization can provide a substantial gain in the link budget and allow the WS and/or Access Point (AP) to be physically mounted on the aircraft without regard to matching the AP and WS linear polarized antenna orientation. To quantify the benefits of circular polarization for the WRS application, including superior propagation and penetration with less susceptibility to outside interference and multi-path signals, the inventors have performed several experiments in the laboratory and in the field, such as inside the aircraft fuselage, that show the advantages of circularly polarized antennas over linearly polarized antennas.
Circular polarization in the context of wireless transmission is linked to the polarization antenna which can be typically defined as a transducer that converts radio frequency electric current into electromagnetic waves that are then radiated into space and vice versa. The polarization type is defined by the electric field (i.e., E) plane that determines the orientation of the radio wave.
An antenna with linear polarization radiates only one plane that contains the line defined by the direction of wave propagation. An antenna with circular polarization, however, radiates in a circular motion in the plane of polarization at the speed of one revolution per radio wave period. In contrast to a horizontally or vertically polarized antenna, where propagation is strictly in one direction, as shown in FIG. 1(a) (horizontal polarization) and in FIG. 1(b) (vertical polarization), a circularly polarized antenna radiates energy in both horizontal and vertical planes as well as in all planes in between. FIGS. 1(a)-1(d) illustrate the differences between linear and circular polarizations. The wave propagation illustrated in FIGS. 1(c)-1(d) is of a Right-Hand-Circular (RHC) type.
Furthermore, the rotation sense of the radio makes the difference between two classes of circularly polarized antennas, namely, Right-Hand-Circular (RHC) and Left-Hand-Circular (LHC) type (counter clockwise direction). For good results and to avoid any dB loss (that could exceed 10 dB), circularly polarized antennas at both the transmission node and the reception node must have the same sense of polarization (i.e., both of RHC type or both of LHC type).
In the event that the radio wave strikes a smooth reflective surface, the wave may incur a 180-degree phase shift, creating what is known as a “mirror image reflection.” In this case, circular polarization can be advantageous because the reflected wave from a circularly polarized antenna would have a different sense than the direct wave and, thus, fading originating from these types of reflections is minimized by using circular polarization.
Linear antennas are typically easier to produce than circularly polarized antennas. FIG. 2 shows a prior art example of producing circularly polarized waves out of linearly polarized waves using a quarterwave plate 50. Most systems today use either linear polarized (LP) to linear polarized (LP), or circular polarized (CP) to circular polarized (CP) to minimize polarization mismatch losses and depolarization. In some other situations when having better fading statistics is more important, mixing CP with LP could be a better alternative than using the same polarization at both ends of the transmission. However, such a mix comes at the expense of 3 dB loss in antenna gain.